James G. Speight - Encyclopedia of Renewable Energy

See also: Algae, Algae Fuels.

Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds, which are photosynthetic but lack the many distinct organs found in land plants. Microalgae are organisms that are less than 0.4 mm in diameter and include the diatoms and cyanobacteria and are capable of photosynthesis. Macroalgae are organisms such as seaweed.

Algae are not highly differentiated in the way that plants are, and they lack true roots, stems and leaves, and a vascular system to circulate water and nutrients throughout their bodies. They can exist as single, microscopic cells; they can be macroscopic and multicellular; live in colonies; or take on a leafy appearance as in the case of seaweeds.

The general term algae includes prokaryotic organisms cyanobacteria, also known as blue-green algae, as well as eukaryotic organisms (all other algal species). Algae lack the various structures that characterize land plants, such as leaves, roots, and other organs that are found in vascular plants. Many algae are photoautotrophic, although some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon. Some unicellular species rely entirely on external energy sources and have limited or no photosynthetic apparatus.

The majority of algae live in aquatic habitats and these organisms can thrive in freshwater lakes or in saltwater oceans. They can also endure a range of temperatures, oxygen or carbon dioxide concentrations, acidity, and turbidity. An important contribution of algae to the environment and well-being is the generation of oxygen through photosynthesis.

Algal biofuels are a promising replacement for fossil fuels. All algae have the ability to produce energy-rich oils, and several microalgal species naturally accumulate high levels of oil in their dry mass. Moreover, algae are found in diverse habitats and can reproduce quickly and also efficiently use carbon dioxide. Algae help to keep atmospheric carbon dioxide levels stable by storing carbon dioxide in organic materials that include crude oil and inorganic carbonate rocks. Green algae, diatoms, and cyanobacteria are just some of the microalgal species that are considered good candidates for the production of biofuel.

See also: Algae Fuel, Aquatic Plants, Biomass.

Algae fuel is a biofuel that is derived from algae, which are photosynthetic, eukaryotic, plant-like organisms that use chlorophyll in capturing light energy, but lack characteristic plant structures such as leaves, roots, flowers, vascular tissue, and seeds. The production of algae to harvest oil for biofuels has not yet been undertaken on a commercial scale. But algae potentially can be grown commercially in environments such as algae ponds at wastewater treatment plants and the oil extracted from the algae and processed into biofuels. During photosynthesis, algae and other photosynthetic organisms capture carbon dioxide.

The benefits of algal biofuel are that it can be produced industrially, thereby obviating the use of arable land and food crops (such as soy, palm, and canola), and that it has a high yield of oil when compared to other sources of biofuel. Thus, algaculture, unlike food crop-based biofuels, does not entail a decrease in food production, since it requires neither farmland nor fresh water.

Algae can produce up to 60% of their biomass in the form of oil. Because the cells grow in aqueous suspension where they have more efficient access to water, carbon dioxide, and dissolved nutrients, microalgae are capable of producing large amounts of biomass and usable oil in either high rate algal ponds or photobioreactors. This oil can then be turned into biodiesel. In fact, seaweeds, which are macroscopic, multicellular marine algae, may offer a particular useful source of biofuels, since they lack lignin and likewise do not require land, fresh water, or fertilizer. One complication is that approximately one-third of the sugars in seaweed take the form of alginate and microbes have not been able to convert it into ethanol.

In the process of oil production, the algae is harvested from the growing process as algae paste after which water is removed by heat drying or de-watering presses. Centrifuges are also another way in which the algae past can be de-watered. The finished product is algae oil in a form that is then suitable for use in the transesterification process to make biodiesel fuel.

The production of biofuel from algae does not reduce atmospheric carbon dioxide (CO 2), because any carbon dioxide taken out of the atmosphere by the algae is returned when the biofuels are burned. They do however eliminate the introduction of new carbon dioxide by displacing fossil hydrocarbon fuels. Also, algal fuels do not affect fresh water resources and they can be produced using ocean and wastewater. Algal fuels are also biodegradable and relatively harmless to the environment if spilled.

Open-pond systems for the most part have been given up for the cultivation of algae with high-oil content. Open systems using a monoculture are also vulnerable to viral infection. The energy that a high-oil strain invests into the production of oil is energy that is not invested into the production of proteins or carbohydrates, usually resulting in the species being less hardy, or having a slower growth rate. Algal species with lower oil content, not having to divert their energies away from growth, have an easier time in the harsher conditions of an open system.

The preference toward microalgae is due largely to its less complex structure, fast growth rate, and high oil content (for some species). Some commercial interests into large scale algal-cultivation systems are looking to tie in to existing infrastructures, such as coal power plants or sewage treatment facilities. This approach not only provides the raw materials for the system, such as carbon dioxide and nutrients; it also changes those wastes into resources. However, there is interest in using seaweed for biofuels, probably due to the high availability of this resource.

Algae fuel is also very appealing in terms of its emissions as well. The combustion of algae fuel produces less carbon monoxide, unburned hydrocarbons, and harmful pollutants compared to crude oil-derived diesel fuel as well as emits no sulfur oxides. Replacing fossil fuels with algae could substantially reduce carbon dioxide emissions.

In addition, microalgae has a strong impact on wastewater, and systems for producing microalgae have the ability of being able to use saline waste, as well as carbon dioxide streams, as an energy source. This is because the algae from microalgae bioreactors is capable of capturing organic compounds and heavy metal contaminants in wastewater. As a result, the production of algae has the side effect of being able to recycle formerly unusable water. In fact, not only does this process clean waste water, but it also recovers phosphorus from the waste water. Phosphorus is a highly limited resource, so much so that the last reserves of phosphorus are estimated to have already been depleted.

Biogasoline is gasoline produced from biomass such as algae. Like traditionally produced gasoline, the constituents contain between 6 carbon atoms (hexane, C 6H 14) and 12 (dodecane, C 12H 26) carbon atoms per molecule and can be used in internal-combustion engines. Biogasoline is chemically different from biobutanol and bioethanol, as these are alcohols, not hydrocarbons.

See also: Algae, Aquatic Plants, Biomass.

Algae fuels can be recovered by three processes: (i) physical extraction or (ii) chemical extraction, and (iii) enzymatic extraction.